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Documentation: admin-guide: cec.rst: document NTP issue

Message ID 4a188eb2-19c4-d9b3-634e-9dd96cc5cac9@xs4all.nl (mailing list archive)
State New, archived
Headers show
Series Documentation: admin-guide: cec.rst: document NTP issue | expand

Commit Message

Hans Verkuil March 27, 2023, 8:32 a.m. UTC 
The CEC pin framework is affected by NTP daemons speeding up or slowing
down the system clock. Document this and explain how to fix this for
chronyd.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
---
diff mbox series

Patch

diff --git a/Documentation/admin-guide/media/cec.rst b/Documentation/admin-guide/media/cec.rst
index 656dec7e0182..6b30e355cf23 100644
--- a/Documentation/admin-guide/media/cec.rst
+++ b/Documentation/admin-guide/media/cec.rst
@@ -55,6 +55,15 @@  Miscellaneous:
   you can control the CEC line through this driver. This supports error
   injection as well.

+- cec-gpio and Allwinner A10 (or any other driver that uses the CEC pin
+  framework to drive the CEC pin directly): the CEC pin framework uses
+  high-resolution timers. These timers are affected by NTP daemons that
+  speed up or slow down the clock to sync with the official time. The
+  chronyd server will by default increase or decrease the clock by
+  1/12th. This will cause the CEC timings to go out of spec. To fix this,
+  add a 'maxslewrate 40000' line to chronyd.conf. This limits the clock
+  frequency change to 1/25th, which keeps the CEC timings within spec.
+

 Utilities
 =========